The main purpose of the study will be to determine whether the thymidine labelling index could provide any significant information regarding prognosis in breast carcinoma, and to what extent the TLI could identify those patients that could have an early relapse. Until now, histologic type of the tumor and steroid receptor status have played an important role in the planning of adjuvant therapy, but we believe that the proliferation characteristics of tumor cells may also be a critical determinant since nuclear DNA contents in populations of breast cancer cells have been proven to have prognostic significance. The thymidine labelling technique relies on the uptake of titrated thymidine by replicating cells which is then detectable by autoradiography. We will investigate the thymidine labelling index range for different histologic types of breast cancer, tumors of patients that have received radiation and/or chemotherapy; the changes in TLI of residual normal breast tissue and the effect of hormonal manipulation in TLI. The rapid processing of autoradiographs allows cell kinetic data for clinical evaluation in three days, which may be a useful adjunctive diagnostic tool on selected cases.